In recent years, a service for transmitting a speech coded data by PTT communications through a packet communication network is started. This is a communication form among terminals forming a group where the speech coded data from a terminal obtaining a right of statement is copied in a PTT server and is transmitted to another terminal participating in the group.
In a case where such PTT communication is carried out, a call connection processing is generally carried out by SIP (Session Initiation Protocol)/SDP (session Description Protocol) and a transmission destination of the coded data and information related to the coded data are notified from a PTT terminal to the PTT server. In addition, a medium coded data is transmitted and received with RTP (Real-time Transport Protocol) (see [Non-Patent Document 1] Handley, M., Schulzrinne, H., Schooler, E., Rosenberg, J., “SIP: Session Initiation Protocol”, RFC 2543, March 1999, Internet URL <http://www.ietf.org/rfc2543.txt>, [Non-Patent Document 2] Handley, M., Jacobson, V., Perkins, C., “SDP: Session Description Protocol”, RFC 4566, July 2006, Internet URL <http://www.ietf.org/rfc/rfc4566.txt>, and [Non-Patent Document 3] Schuizrinne, H., Casner, S., Fedrick, R., Jacobson, V., “RTP: A Transport Protocol for Real-Time Applications”, RFC 3550, July 2003, Internet URL <http://www.ietf.org/rfc/rfc3550.txt>.
Under present circumstances, only a PTT service due to speech is started. However, with expanding a network band in the future, it is considered that a PTT service with a multimedia including video (picture) or image is developed and more fleshed information is exchanged between users.
However, in a case where specifications of PTT terminals are not uniform, although the PTT server copies the medium coded data transmitted from an individual terminal (a PTT terminal of a transmission source) to transmit another terminal participating in the group (a PTT terminal of a transmission destination), there is a possibility that the PTT terminal (the PTT terminal of the transmission destination) which cannot decode correctly exists. For example, if data is a speech coded data, there is a case that the PTT terminal where a coding method, a bit rate, and a mode to support are different exists in the group. In addition, if data is an image coded data, there is a case that the PTT terminal (the PTT terminal of the transmission source) that produces the image coded data having an image resolution and a frame rate which cannot decode by other PTT terminals (the PTT terminals of transmission destination) exists in the group. Although the PTT servers copies the speech coded data having a different coding method or the image code data that cannot decode due to a high image resolution to transmit them, the PTT terminal (PTT terminal of the transmission destination) receiving them cannot decode such a speech coded data or such an image coded data.
Furthermore, for example, attention will be directed to an image coding method such as MPEG (Moving Picture Experts Group)-4. It will be assumed that a coded data transmitted from a PTT server for transmission source has coding parameters such an image resolution, a frame rate, and a bit rate that are within the confines of the support in a PTT terminal for transmission destination. In addition, it will be assumed that the coded data transmitted from the PTT server for the transmission source has a coding tool such as data partitioning and reversible variable-length code that are different from specification of the PTT terminal for the transmission destination. In this event, the PTT terminal for the transmission destination may not decode received coded data. Likewise, it will be presumed that the received coded data has resolution of time information that is different from encoding specification obtained by call control processing. In this event, the PTT terminal for the transmission destination may not correctly decode the received coded data. This is because an image decoder generally initializes the coding tool and the time resolution to be used using information preliminarily obtained by call control. Similarly, as regards speech, when payload option is packed by option different from information obtained by the call control processing, the PTT terminal for the transmission destination may not correctly decompress the coded data.
Under the circumstances, it is necessary for the PTT server to convert a coding format, an image resolution, and so on of the coded data to be transmitted for each of PTT terminals having different coding specification and decoding ability. As a result, when the PTT server replies transmission of an image coded data, it results in increasing a processing amount required for conversion of the image coded data. In addition, in a tandem conversion serving as a general conversion, degradation of quality due to the conversion is not avoided.
On the other hand, Japanese Unexamined Patent Application Publication JP-A 2006-203682 (which corresponds to United States Patent Application Publication US 2006/0165180 A1), which will later be referred to as “a patent document 1”, describes a method of reducing a computing amount and of maintaining quality of an image coding tool conversion and resolution conversion of time information. However, a congenially computing amount is required when the above-mentioned conversion processing is carried out for the respective PTT terminals. In addition, there is a possibility that the number of processing channels per PTT server is restricted.
In addition, Japanese Unexamined Patent Application Publication JP-A 2006-67124 (which corresponds to United States Patent Application Publication US 2006/0045363 A1), which will later be referred to as “a patent document 2”, discloses a system which is capable of mutually transferring images among a plurality of clients even if coding methods, coding settings, coding options, or states of transmission path are different from each other.
Furthermore, Japanese Unexamined Patent Application Publication JP-A 2005-94800, which will later be referred to as “a patent document 3”, discloses a moving picture re-encoding apparatus which has much flexibility in performing a special replay, an overlay processing and etc. that enable a digital connection between digital video systems employing different method, and which has high image quality and it at low cost.
In addition, WO2003/102949, which will later be referred to as “a patent document 4”, discloses a data duplicate recording device which is capable, when data of a first recording medium is copied to recode in a second recording medium different from the first recording medium, of correctly reproducing data from the second recording medium. In the data duplicate recording device disclosed in the patent document 4, the data recorded in the first recording medium is a first coded data which is compression coded with a first coding method. The data duplicate recording device comprises a coding method judgment unit for judging whether or not the first coding method is a second coding method matched to the second recording medium, a coded data conversion unit for converting the first coded data read out of the first recording medium into a second coded data which is compression coded with the second coding method when judgment is made where the first coding method is not the second coding method, a recording unit for recording, in the second recording medium, the second coded data obtained by the coded data conversion unit.